1. Field of the Invention
The present invention relates to a control method used in an image forming apparatus that employs an electrophotographic process, an electrostatic recording process or the like and to an image forming apparatus using the control method. The present invention is applicable to image forming apparatuses such as a copying machine, a printer and a facsimile machine.
2. Related Background Art
Conventionally, there has been proposed an image forming apparatus that repeats a process of transferring a toner image, which is formed on a photosensitive drum using developer consisting of carrier and toner, to paper attracted and borne on a transfer drum, thereby forming a full color image on the paper.
In such an image forming apparatus, a weight ratio of toner and carrier contained in a developing device changes by repeated development operations and supply of toner to the developing device. In order to grasp the change, a density detecting mechanism for detecting information corresponding to a weight ratio of toner and carrier is provided in the image forming apparatus.
For example, a patch sensor is provided in a position opposed to a transfer sheet constituting the transfer drum, and a density of a patch-shaped developed image for density detection (patch) transferred onto the transfer sheet is detected by this sensor.
In addition, a weight ratio of toner and carrier in the developing device, that is, a supplied toner amount is controlled such that a detected density of a patch image is maintained constant.
Such a method of forming a patch image on the transfer drum and a density detecting mechanism for a patch image will be further described.
In the image forming apparatus, a reference image generating circuit having a signal level corresponding to a predetermined density is provided as one of image control means. In a patch image forming process, a laser beam is emitted according to a reference image signal from the reference image generating circuit to scan a surface of the photosensitive drum. Consequently, an electrostatic latent image for density detection (reference electrostatic latent image) corresponding to the predetermined density is formed on the photosensitive drum. This reference electrostatic latent image is developed by the developing device, whereby a patch image is formed. Thereafter, this patch image is transferred to the transfer sheet by a transfer charger.
In addition, conventionally, there has been proposed a patch sensor 13 as shown in FIG. 1 as a sensor for detecting a density of such a patch image. The patch sensor 13 uses a near infrared LED and a photodiode (PD) as a light emitting element and a light receiving element, respectively, to detect a density from a regular reflection light amount and a diffuse reflection light amount that are obtained from a developed image (toner image) 200 visualized on a transfer sheet 5f. A method for the detection will be described below.
The patch sensor 13 is constituted by PDs 13e, 13f and 13g and prisms 13h and 13i. Light irradiated by an LED 13c is split into a component vibrating in a vertical direction with respect to an incident surface (s-wave light) and a component vibrating in a parallel direction with respect to the incident surface (p-wave light).
The s-wave light is irradiated on the PD 13e in the vicinity of the LED 13c and the p-wave light is irradiated on a toner surface. The p-wave light, which is incident on a surface to be a background such as transfer sheet in detecting a density, is generally reflected regularly and is transmitted through the prism 13i to be incident on the PD 13f with regular reflection light as a p-wave. The p-wave light irradiated on the toner surface, which is a patch image, is diffusely reflected and split into a p-wave and an s-wave with a part of the p-wave light turning into the s-wave. Transmitted through the prism 13i, the p-wave is incident on the PD 13f and detected as regular reflection light, and the s-wave is incident on the PD 13g and detected as diffuse reflection light. Thus, the PD 13f functions as regular reflection light amount detecting means and the PD 13g functions as diffuse reflection light amount detecting means.
Here, outputs of each of the p-wave by the PD 13f and the s-wave by the PD 13g with respect to patch image densities are shown in FIG. 21A. According to FIG. 21A, a diffuse reflection component is considered to be actually incident on the PD 13f as well. Thus, a real regular reflection output as shown in FIG. 21B is obtained by deducting the output of the s-wave by the PD 13g multiplied by a certain correction coefficient from the output of the p-wave by the PD 13f, that is, from the following expression. The correction coefficient is a predetermined fixed value.    Corrected output=“Regular reflection light amount    (p-wave) output”−“Diffuse reflection light amount    (s-wave) output”×Correction coefficient
A corrected output obtained in this way is converted according to a graph of FIG. 21B and detected as a patch image density. Based on this result of patch image density detection, a weight ratio of toner and carrier (toner supply amount) and operating conditions (applied bias, etc.) of a charger taking part in image formation, a developing device and a transfer charger, that is, image forming conditions are controlled such that an image is formed with an accurate density.
However, in the patch sensor 13 and a method of using the same, outputs from the respective patch sensors 13 may vary due to an individual difference of the patch sensor 13 or attachment accuracy or the like in attaching the patch sensors 13 to the image forming apparatus despite the fact that densities of formed patch images are the same.
As a result of inspection of causes of this variation, it was found by the inventor that, if there are problems in the individual difference of the patch sensor 13 or the attachment accuracy in attaching the patch sensors 13 to the image forming apparatus, since, for example, outputs of the two PDs 13f and 13g in sensing a predetermined reference plate changes and an output ratio of the two PDs 13f and 13g changes accordingly, the above-described corrected output changes.